It is known that treatment of mixtures of soluble aluminum and magnesium salts with aqueous blends of sodium hydroxide and carbonate will precipitate solids having an approximate empirical formula: EQU Mg.sub.0.7 Al.sub.0.3 (OH).sub.2 (CO.sub.3).sub.0.15 .times.H.sub.2 O
These are referred to as synthetic hydrotalcites from analogy to the naturally occurring mineral hydrotalcite having the approximate empirical formula: EQU Mg.sub.0.75 Al.sub.0.25 (OH).sub.2 (CO.sub.3).sub.0.125 .times.H.sub.2 O
They are derivatives of the brucite magnesium hydroxide structure, i.e., flat plates of magnesium coordinated to four oxygens in a square planar arrangement. A good discussion is given by Reichle, "Synthesis of Anionic Clay Minerals (Mixed Metal Hydroxides, Hydrotalcite)" Solid States Ionics, Vol. 22, 135-14 (1986). Partial replacement of magnesium in the brucite magnesium hydroxide structure by aluminum leads to an extra positive valence for each Al included. Thus each Al for Mg inclusion will accommodate one monovalent or 1/2 of a divalent anion. The extra anions are sandwiched between the flat magnesium/aluminum brucite layers and can be exchanged for other anions. This reaction is believed to be driven to the right by the acidic strength of the parent acid of the anion. For instance, it is likely that HCl will cause displacement of hydroxide or carbonate to form water or carbon dioxide, for example: ##STR1##
Aluminum cannot be substituted for magnesium past a certain point or the hydrotalcite structure will collapse to a range of other mixed Mg/Al hydroxide/carbonates not having the brucite structure. It is the brucite structure that traps chloride efficiently and largely irreversibly at temperatures below 250-300.degree. C.
The methods that can be used to make synthetic hydrotalcites are described by Miyata, et al., U.S. Pat. Nos. 4,299,759 and 4,085,088. These patents describe hydrotalcites having a BET specific surface area of not more than 30 m.sup.2 /g, preferably not more than 15-20 m.sup.2 /g as heat stabilizers for halogenated polymers. These patents also state that hydrotalcites having a specific surface area of at least, or more than, about 50 m.sup.2 /g should be avoided because they tend to accelerate the dechlorination of the resins under heat to cause decomposition, blackening or foaming of the resins. It is also stated that these hydrotalcites have poor dispersibility in the resins and adversely affect properties during molding and the appearance of the molded products. The useful hydrotalcites are prepared, according to Miyata, either by precipitation at room temperature and ambient pressure, followed by heat treatment with water at 150.degree.-250.degree. C. under pressure; or precipitation in this temperature range under pressure.